Engine parameters such as air-fuel ratio (AFR) can be controlled to ensure improved engine performance leading to effective use of an exhaust catalyst and reduced exhaust emissions. In particular, if engine exhaust gases are not rich or lean as expected due to engine air-fuel ratio variation between an engine's cylinders, engine emissions may degrade. In addition, there may be torque imbalance between the engine cylinders which can result in NVH issues.
One way to determine air-fuel ratio variation between engine cylinders is to sense engine exhaust gases via an oxygen sensor. Additionally or optionally crankshaft acceleration may be estimated at a desired AFR. Fuel and/or charge air parameters may then be adjusted based on the variation to produce an air-fuel mixture at a target air-fuel ratio. However, the oxygen sensor may be exposed to exhaust gases that are a combination of gases from different engine cylinders. Therefore, it may be difficult to accurately determine air-fuel variations between different engine cylinders. Further, engine exhaust system geometry for cylinders having a large number of cylinders may bias sensor readings toward output of one cylinder more than other cylinders. Consequently, it may be even more difficult to determine air-fuel imbalance for engines having more than a few cylinders.
Furthermore, in dual fuel injection systems where the engine is configured with hardware for each of direct injection and port fuel injection (PFDI systems), it may be difficult to differentiate between DI and PFI induced air-fuel ratio imbalance. This is due to both injectors being active during the monitoring. In addition, purging of fuel vapors and use of positive crankcase ventilation (PCV) can further corrupt oxygen sensor outputs when scheduling a fuel pulse through an injector, requiring complicated calculations to compensate for the ingested hydrocarbons. If the AFR monitoring is scheduled during conditions when purge or PCV is disabled, there may be limited opportunities for AFR monitoring. On the other hand, if purge is disabled to complete AFR monitoring, a fuel vapor canister may not be effectively cleaned, leading to emissions issues.
The inventors herein have recognized the shortcomings discussed above and have developed a method for determining air-fuel ratio imbalance and injector error in engine cylinders taking into account AFR variations among cylinder groups. In one example, AFR imbalance may be determined by a method for an engine, comprising: during a deceleration fuel shut-off (DFSO) event where all cylinders of an engine are deactivated, sequentially firing each cylinder of a cylinder group, each cylinder fueled via consecutive first and second fuel pulses of differing fuel pulse width from an injector; and based on a lambda deviation between the first and second pulses, learning a fuel error for the injector and an air-fuel ratio imbalance for each cylinder.
In this way, AFR monitoring may be performed independent of purge or PCV hydrocarbons, and while better differentiating errors from distinct injectors.
In one example, AFR errors may be learned during deceleration fuel shut-off conditions (DFSO), a period characterized by lower driver demand torque where the engine is in motion, and spark and fuel supply to one or more cylinders is cut-off. During the DFSO conditions, a cylinder group may be sequentially fired, with at least two consecutive fuel pulses of different pulse widths delivered to each cylinder. A change in AFR corresponding to each pulse width may be learned. An air-fuel ratio imbalance for the injector of the given cylinder group is then determined based on a deviation from a maximum lean air-fuel ratio measured under DFSO conditions. In particular, an engine controller may learn a first change in AFR following the first fuel pulse relative to a second change in AFR following the second fuel pulse. This may be compared to a pulse width of the first fuel pulse relative to the pulse width of the second fuel pulse to determine the injector error. Alternatively the injector error following each pulse may be determined based on a change in crankshaft acceleration following each fuel pulse. Assuming that the quantity of air charge and excess fuel vapors purged to the engine intake during the monitoring remain constant, errors due to the ingestion of purge or PCV hydrocarbons are not introduced.
The approach described here may confer several advantages. For example, the method provides improved capability for learning air-fuel ratio imbalance and allows better detection of injector error among cylinder groups. Consequently, the approach ensures improved fuel efficiency and reduced emissions. In addition, the method automatically compensates for air-fuel ratio imbalance associated with purge and PCV. By rendering the learning independent of the presence of purge or PCV fuel vapors, injector learning can be performed over a wider range of engine operating conditions, and without compromising canister purge efficiency. The technical effect of learning air-fuel ratio imbalance and injector error among cylinder groups is AFR errors may better learned, improving exhaust emissions and engine performance.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.